The invention relates to index guided, inner stripe laser diode structures.
Inner stripe laser diode structures have been commonly used in both red AlGaInP and infrared AlGaAs laser diodes. Inner stripe laser diode structures provide a convenient means of achieving low threshold, single mode laser diodes. An inner stripe laser diode structure requires two epitaxial growth steps. The first epitaxial growth step typically involves growth of the lower cladding layer, the active region, a portion of the upper cladding layer and an n-type blocking layer. Following the etching away of the n-blocking layer in a narrow stripe, the remaining portion of the n-cladding layer is grown. In operation, the injection current path is defined by the etched stripe opening in the n-blocking layer, even though the p-metal contact pad may be significantly wider than the stripe.
The current-blocking layer is placed close to the active region, typically being at about a 100-200 nm separation from the boundary of the active region into the upper-cladding layer. Due to the relative ease of creating a 1-2 xcexcm wide stripe in the n-blocking layer, very low threshold current lasers can be fabricated. It is much easier to form a 2 xcexcm wide stripe in this self-aligned structure in comparison with, for example, the similarly narrow ridge waveguide laser because a ridge waveguide structure requires that a narrow contact stripe, typically 1-1.5 xcexcm, be carefully aligned on the top of the ridge structure. Very narrow inner stripe laser diodes offer improved heat dissipation because lateral heat spreading is enhanced as the width of the laser stripe is reduced. Hence, when a new semiconductor laser material system is developed, the inner stripe structure is often the first structure used to achieve a single mode laser.
Although the inner stripe laser diode structure is advantageous to achieving low threshold, single mode operation, the resulting beam quality is relatively poor and unsuitable for many important applications. The beam quality is relatively poor because no lateral positive index guiding is provided by the inner stripe laser diode structure. Instead, a highly astigmatic beam is generated because the inner laser stripe structure is gain-guided. While astigmatism is correctable using cylindrical optics, the lateral beam divergence and astigmatism may vary with drive current.
Nitride inner stripe laser diodes with a current blocking layer are disclosed in U.S. Pat. No. 5,974,069 by Tanaka et al. Tanaka et al. disclose current blocking layers made from materials including those selected from a group consisting of AlyGa1xe2x88x92xN (0 less than y=1), SiO2, Si3N4 and Al2O3.
FIG. 1a shows the lateral index step as a function of the thickness of first upper cladding layer 5 for two cladding layer laser diode structure 11 shown in FIG. 1b which is similar to that of Tanaka et al. Note that Tanaka et al. disclose a first upper cladding layer with a thickness of at least 100 nm which limits the lateral index, An, to no greater than 4xc3x9710xe2x88x923. FIG. 1b shows active region 4 beneath first upper cladding layer 5 which supports current blocking layer 6 covered by second upper cladding layer 7. Curve 50 shows that as the thickness of first upper cladding layer 5 increases for two cladding layer structure 11, the lateral index step (and the resulting optical confinement) drops from an initial lateral index step of about 11xc3x9710xe2x88x923 at zero thickness.
The structure of an inner stripe laser may be modified to produce lateral index guiding and provide the beam quality necessary for printing and optical storage. The modified inner stripe laser structure allows for excellent beam quality and modal discrimination while retaining the benefits of the basic inner stripe laser structure. The modified inner stripe structure is applicable to nitride laser diode structures and other material systems which are relatively insensitive to regrowth interfaces close to the active region. In AlGaAs and AlGaInP laser systems, for example, the defect states associated with a regrowth interface close to the active region are sufficient to inhibit lasing.
The modified inner stripe laser structure involves an epitaxial growth of a conventional inner stripe laser structure including a partial upper waveguide layer. A current blocking layer is then grown on the partial upper waveguide layer with a narrow stripe subsequently opened in the blocking layer. Following definition of the narrow stripe, an epitaxial regrowth is performed to complete the upper waveguide layer along with the cladding and contact layers. Finally, the structure is processed in the standard manner, including contact metallization and mirror formation.
Because regrowth for the modified inner stripe laser structure starts with the upper waveguide layer instead of proceeding directly to regrowth of the upper cladding layer, a positive lateral index guide may be created. By starting regrowth with the upper waveguide, the thickness of the waveguide in the narrow stripe region is made greater than the thickness of the wing regions of the waveguide. The wing regions of the upper waveguide are the waveguide regions beneath the blocking layer next to the active region. The difference in thickness functions to produce a lateral index step. The strength of the lateral index waveguiding depends in part on how close the blocking layer is placed to the active region with closer placement providing better lateral index guiding. The current blocking layer may be n-type or insulating material and materials that may be used for the current blocking layer include AlGaN, AlN, SiO2, SiON and Si3N4. Typical values of the lateral index step obtained in accordance with the invention are about 20xc3x9710xe2x88x923.